A fuel cell uses a chemical conversion of a fuel into water with the aid of oxygen to generate electrical energy. For this purpose, the fuel cell includes at least one so-called membrane electrode assembly (MEA) as a core component, which is an assembly of an ion-conducting, often proton-conducting, membrane and electrodes, an anode and a cathode, situated on both sides of the membrane. In addition, gas diffusion layers (GDL) may be situated on both sides of the membrane electrode assembly, on the sides of the electrodes facing away from the membrane.
The fuel cell is generally formed from a large number of membrane electrode assemblies arranged in a stack, their electrical powers adding up during an operation of the fuel cell. Bipolar plates, also referred to as flow field plates, are generally situated between the individual membrane electrode assemblies and ensure a supply of operating media, so-called reactants, to membrane electrode assemblies, i.e., the individual cells of the fuel cell and are usually also used for cooling. In addition, the bipolar plates ensure an electrically conductive contact with the membrane electrode assemblies.
During an operation of an individual cell, the fuel, a so-called anode operating medium, in particular hydrogen (H2) or a hydrogen-containing gas mixture, is supplied via a flow field of the bipolar plate, which is open on the anode side, to the anode, where an electrochemical oxidation of H2 to 2H+ takes place with the discharge of electrons (2e−). A water-bound or water-free transfer of protons (H+) from an anode space into a cathode space takes place through a membrane or an electrolyte, which separates and electrically insulates the reaction spaces from each other in a gas-tight manner. The electrons provided at the anode are supplied to the cathode via an electric line and an electric consumer (electric motor).
A so-called cathode operating medium, in particular an oxygen (O2) or an oxygen-containing gas mixture, for example air, is supplied to the cathode via a flow field of the bipolar plates, which is open on the cathode side, so that a reduction from O2 to 2O2− takes place with the absorption of the electrons. At the same time, oxygen anions (O2−) formed in the cathode space react with the protons transferred through the membrane, forming water.
To supply a fuel cell stack, hereinafter also referred to mainly as a fuel cell, with operating media, the fuel cell stack includes an anode supply system, on the one hand, and a cathode supply system, on the other hand. The anode supply system has an anode supply path for supplying the anode operating medium into the anode spaces of the fuel cell and an anode exhaust gas path for discharging an anode exhaust gas out of the anode spaces. Similarly, the cathode supply system has a cathode supply path for supplying the cathode operating medium into the cathode spaces of the fuel cell and a cathode exhaust gas path for discharging a cathode exhaust gas out of the cathode spaces.
The explanations below refer to the prior art illustrated in FIGS. 2 and 3. In cathode supply system 30, the cathode of fuel cell 10 may be fluid-mechanically separated from surroundings 2 with the aid of a shutoff valve 310 in cathode supply path 31 and with the aid of a shutoff valve 320 in cathode exhaust gas path 32. Anode supply system 20 and cathode supply system 30 are furthermore fluid-mechanically connectable with the aid of a purge valve and a separator valve in corresponding lines between anode supply system 20 and cathode supply system 30.
In FIG. 2, separator valve 401 is designed as an NO (normally open) overflow valve 401, i.e., an overflow valve which is open when it is not being energized or activated, i.e., for example, when the fuel cell is deactivated, to avoid a critical overpressure or a critical pressure difference between the anode and the cathode of fuel cell 10 when fuel cell system 1 is deactivated. In addition, hydrogen is supplied to the cathode with the aid of NO overflow valve 401 to protect it against harmful air-air startups of fuel cell 10.
Another option according to the prior art for avoiding a critical overpressure is to design drain valve 401 as an NO overflow valve 401, which is illustrated in FIG. 3, and to facilitate a pressure compensation and a supply of hydrogen to the cathode via NO overflow valve 401. The problem with these two approaches is that hydrogen is not homogeneously distributed between the anode and the cathode in the fluid-mechanically blocked part of the fuel cell during and after a deactivation of the fuel cell, since a flow of the hydrogen takes place almost exclusively via the corresponding open valve and not via the membrane of the fuel cell.
DE 10 2006 035 851 B4 discloses a fuel cell system, which includes an anode supply path and a cathode supply path for supplying hydrogen and air to a fuel cell. The fuel cell system furthermore includes an anode exhaust gas path and a cathode exhaust gas path. The cathode supply path may be brought into fluid communication with the anode supply path via a line and an air supply valve situated in the line. The air supply valve is opened for an anode purging process, so that cathode-side air may enter on an anode side of the fuel cell. If an anode purging process does not take place, the air supply valve is closed, independent of pressure.